Cellular Respiration: Glycolysis and the Krebs Cycle

Glycolysis

• Begins with a glucose molecule (6-carbon molecule).
• Glycolysis splits glucose into two pyruvate (pyruvic acid) molecules (3-carbon each).
  • Process does not require oxygen (anaerobic).
  • Net gain of 2 ATPs (uses 2 ATPs, generates 4 ATPs).
  • Produces 2 NADH molecules.
• Occurs in the cytoplasm of the cell.

Mitochondria Structure

• Mitochondria is the "powerhouse" of the cell.
• Consists of an outer membrane and an inner membrane (cristae).
• Inner compartment is called the matrix.

Pyruvate Oxidation

• Prepares pyruvate for the Krebs Cycle.
• Occurs in the mitochondria.
• Pyruvate (3-carbon) is oxidized to form acetyl-CoA (2-carbon).
  • Cleaves one carbon, releasing CO2.
  • Produces NADH from NAD+.

The Krebs Cycle (Citric Acid Cycle)

• Takes place in the mitochondrial matrix.
• Acetyl-CoA (2-carbon) combines with oxaloacetic acid (4-carbon) to form citric acid (6-carbon).
• Citric acid is oxidized in a series of steps to regenerate oxaloacetic acid.
  • Releases 2 CO2 molecules per cycle.
  • Generates ATP, 3 NADH, and 1 FADH2 per cycle.
• Each glucose molecule results in two cycles (due to two pyruvate molecules from glycolysis).

Energy Production Summary

• Glycolysis: 2 ATP, 2 NADH.
• Pyruvate Oxidation (per glucose): 2 NADH.
• Krebs Cycle (per glucose): 2 ATP, 6 NADH, 2 FADH2.

Electron Transport Chain (ETC)

• NADH and FADH2 from glycolysis and Krebs cycle provide electrons for the ETC.
• Each NADH can produce 3 ATPs, and each FADH2 can produce 2 ATPs through oxidative phosphorylation.
• Total ATP production:
  • 4 ATP from glycolysis and Krebs cycle directly.
  • 34 ATP from the electron transport chain.
  • Total: 38 ATP per glucose (theoretical maximum).

Additional Metabolism Notes

• Not only carbohydrates, but proteins and fats can be catabolized.
• Acetyl-CoA acts as a central molecule in metabolism, feeding into the Krebs Cycle regardless of the original nutrient type.